1. Field of the Invention
The invention relates to noise reduction and, more particularly, to a method for reducing fringe interference of light created in a passive cavity defined by partially reflecting optical surfaces in a laser spectroscopy system, where the optical path length of the cavity is varied with a triangular back-and-forth movement.
2. Description of the Related Art
Laser absorption spectroscopy offers high speed and high precision capabilities for detection of numerous trace gas species in gas mixtures at atmospheric pressure with a small cross sensitivity towards other gas components. Tunable diode laser spectrometers are particularly suited to high sensitivity studies, in part because they may be frequency modulated to reduce low frequency laser noise and electronic noise. A typical spectrometer includes a frequency tunable laser for generating a laser beam which passes through a sample cell containing the gas mixture onto an optical detector. The signal received at the optical detector is demodulated to obtain an absorption induced signal.
Unfortunately, the sensitivity is limited by optical noise caused by light that is scattered or reflected by partially reflecting optical surfaces of the sample cell and of other optical elements of the optical system of the laser spectrometer, such as windows or lenses. Parallel optical surfaces form passive optical cavities or etalons which may create the so-called etalon effect when the reflected or scattered light reaches the optical detector and coherently mixes with the primary laser beam. When the laser is frequency or wavelength tuned through the range of the desired absorption signal, the mixing generates a random sinusoidal modulation of the base-line. Here, the period of the sinusoidal fringe depends on the path length difference between the stray light and the main beam. The light impinging on the detector comprises one main component and many stray components of smaller amplitude, thus creating, as the laser wavelength is scanned, a periodic wavelength dependent stationary pattern of several sinusoidal components which, even when very weak, can easily obscure the absorption signal of interest from the sample and thus affect the accuracy of the spectrometer.
The reflections causing the interference fringes are extremely difficult to completely eliminate even with high quality anti-reflection coatings and careful optical alignment. Moreover, the phase of the base line fringes is very sensitive to small variations in the alignment of the optical system when the pattern changes with the ambient temperature of the spectrometer device.
The interference pattern is deterministic rather than random so that normal averaging of the laser scans fails to reduce the interferences. One well-known effective way to reduce the fringes is to vary the path length of the stray components by vibrating the position of an optical element in the laser spectrometer.
U.S. Pat. No. 4,684,258 to Webster describes the insertion of a vibrating Brewster plate between two etalon creating surfaces and thus periodically changing the optical path length of the etalon. U.S. Pat. No. 4,934,816 to Silver et al. describes a similar mechanical approach, where etalon effects in a multipass cell are reduced by introducing a vibrating mirror. In both cases, however, the vibration frequency is asynchronous with the laser modulation frequency so that the fringe pattern due to etalon effects will be averaged out. Moreover, both approaches disclosed in the Webster and Silver et al. patents use a triangular waveform to drive the plate and mirror into oscillation, respectively.
A triangular waveform offers better etalon fringe reduction in comparison to square or sinusoidal waveforms because the time spent by the vibrating element at the turning points is minimized. Unfortunately, this approach has two drawbacks. Firstly, generation of a triangular waveform requires a highly linear electromechanical transducer and imposes high requirements on the electromechanical setup. Secondly, in practice, the vibration amplitude of the optical element has to be more than 30 Free Spectral Ranges (FSRs) or 15 laser wavelengths to obtain a sufficient reduction of the etalon effect. This becomes especially impractical when longer laser wavelengths are used thus imposing higher power consumption and placing a higher demand on the mechanical components (for example, standard piezo-transducers have limited length expansion capabilities). Moreover, unwanted displacement/defocusing of the laser beam may appear when the position of the element oscillates with large amplitudes.
EP 1 927 831 discloses varying the optical path length of the passive optical cavity with a Gaussian (normal) distribution, where the standard deviation is at least one-quarter of the light's wavelength. Thus, compared to triangular modulation which requires vibration amplitudes over several laser wave-lengths, an efficient etalon averaging is obtained already at amplitudes following a Gaussian distribution with a standard deviation slightly above one-quarter wavelength. Another advantage is that, due to the character of noise modulation, there is no need to amplitude and phase control the modulating waveform, thus allowing a much simpler hardware design. On the other hand, the random (noise) modulation contains high frequency components which exert higher acceleration forces on the moving mechanics.